1. Field of the Invention
The invention relates to electrical discharge machining and refers more specifically to structure for and a method of electrical discharge machining of small holes in workpieces with a wire electrode which structure is completely controlled by a Central Processing Unit which may be a programmable microprocessor and which invention includes structure for and the steps of sensing initial sparking between an electrode and a workpiece as a reference point for control of depth of cut, controlling voltage during machining in small increments to control hole size, feeding the electrode to compensate for electrode wear, ultrasonically vibrating one of the wire electrode wire guide and workpiece during cutting, providing a no backlash low friction screw drive for the electrode utilizing a fine pitch precision screw and loose nut with plastic therebetween, removing the tapered end of a worn electrode and eliminating electrode splitting and high velocity flushing.
2. Description of the prior Art
In the past electrical discharge machining has sometimes been partly controlled by a Central Processing Unit including a programmable microprocessor. However, complete control of the electrical discharge machining of small holes with a wire electrode is not believed to have been known.
Also, prior electrical discharge machining of wire electric has accomplished by exactly positioning a workpiece and moving an electrode a predetermine distance from an initial home position for electrode carrying structure. With such structure, the depth of cut depends on the relative position of the workpiece and structure carrying the electrode as well as the length of the electrode. The depth of cut thus changes as the electrode wears. No known electrical discharge machining structure determines wire feed control from an initial position of an electrode on sensing of a first spark at the beginning of a cut.
Electrode feed structure has been utilized in the past to compensate for electrode wear in such equipment. Electrode feed structure of the past has sometimes been by mechanical ratchet structure. Alternatively, fixed electrode feed is known based on clamping and unclamping electrode securing structure periodically. Also, electrode feed is known based on feeding the electrode end to a known position relative to but spaced from the workpiece before each cut.
With such methods of electrical discharge machining and electrode feed the exact position of the electrode tip with respect to the workpiece is seldom known and sometimes results in undesirable cuts.
Further, in the past it has been known to vary voltage in rather large increments to vary electrical discharge machine cutting. No precise hole size regulation due to complete control of electrical discharge machine cutting parameters such as voltage, has been known.
Also, a search of prior patents has not revealed a teaching of ultrasonic vibration of one of the electrode and workpiece during flushing while cutting small holes to improve electrical discharge cutting efficiency. Prior known structures sometimes pulse electrode feed structure to enhance flushing. However, such pulsing is accomplished at frequencies much lower than ultrasonic frequencies.
Further, it is not believed to have been known in the past to ultrasonically vibrate a wire guide to reduce friction between the wire guide and a fragile wire electrode in the wire guide and thus eliminate or greatly reduce compression buckling of the wire electrode and resulting loss of tolerance.
Further, in accordance with the invention, the depth of an electrical discharge machined hole is measured from the initiation of the first spark between an electrode and workpiece considering the position of the electrode at that time. The depth of the hole is thus measured from the actual start of the electrical discharge machining of a workpiece.
Electrode feed is accomplished in accordance with the present invention by clamping an electrode in a first clamp on a slide on an electrical discharge machining head at a rearward location on the electrode and moving the clamp and the electrode relative to the workpiece on the slide. The electrode thus held may be brought to a home or start position with the slide after each electrical discharge machining cut without refeeding the electrode after each electrical discharge machining cut. When it is desired to feed the electrode due to excessive wear of the electrode, another clamp on the electrical discharge machining head is actuated to grip the electrode at a forward location thereon and the slide and the first clamp which has been released are returned to the home position after which the forward location clamp is released and the rearward location clamp is again clamped to the electrode to complete incrementing the wire electrode.
Also, prior known structure has utilized expensive nut and screw or precision ball drives for advancing the electrode toward a workpiece by servo mechanism. It is believed to be unknown in the electrical discharge machining prior art to utilize a screw and nut combination for driving of the electrode wherein backlash is removed from the screw and nut by means of a plastic extruded into the space between the screw and nut of a precision screw and loose nut set.
Also, in the past the cutting of small holes has been hindered by the wearing of the electrode on the cutting end thereof in a tapered manner to provide a subsequent tapered hole. It is not believed to be known in the prior art to square off the end of the wire electrode in electrical discharge machining by wearing it away on electrical discharge cutting initially with a reverse or high wear polarity.
Also, in the past electrical discharge machining of small holes with tungsten wire has at times been greatly hindered by electrodes which split longitudinally during use and contact the sides of the holes being machined and consequently produce objectionable machining of the sides of the holes. It is not believed to have been known to eliminate this splitting of wire electrodes by control of current relative to time of the discharge pulse in electrical discharge machining with a small toroidal series inductor.
Similarly flushing between wire electrodes and workpieces at high velocity has in the past caused displacement of the wire and consequent undesirable hole characteristics.